Simple question: Is there any markdown editor for iOS which supports inline Latex commands (and preview)?
The short answer is "no", I've been looking myself.
TexWriter is a decent LaTex editor and preview but I don't know of a MarkDown one with inline LaTex.
Further, you might want to check out Brett Terpstra's page devoted to iOS editors for a great list of editors and keep an eye out for any that add "LaTex" as an extra feature.
Dropbox paper works on iPads. Typing in $$ will start an inline latex environment.
MWeb on iOS and macOS supports inline
LaTeX Markdown extension. Inline
LaTeX is rendered in the markdown preview.
Markdown Assist: Useful Table insert and LaTeX writing aids.
Markdown syntax: Supports Table, TOC, LaTeX, Fenced code block, Task lists, Footnote, etc.